Polymer phase separation has established a series of bottom-up nanofabrication methods. Owing to the intrinsic immiscibility of most polymer blends, phase separation is typically produced by rapid quenching during spin-casting. However, the full sequence of events that occur during this process is still unclear, especially in the case of ultrathin polymer blend films. Herein, a freestanding method is first introduced to obtain morphological information from the bottom side of the film. We demonstrate that when the thickness of ultrathin film is comparable to the dimensional scale of the phase separation domains, it is feasible to prepare perforated films with uniform nanopores via selective solvent etching. Our results also provide direct evidence that the spinodal decomposition mechanism plays an important role in determining the final morphology within the ultrathin polymer blend films. These findings are of practical value in the fabrication of desired nanostructures by polymer phase separation.
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